More particularly, the invention relates to a structural connecting rod made of composite and its method of manufacture, such as the structural connecting rods used in the production of aeronautical structures because of their favorable structural resistance to weight balance.
A structural connecting rod is an element of elongate form intended to take up traction and/or compression forces along a main axis of the connecting rod that corresponds to the direction in which the connecting rod is elongated.
More often than not, a connecting rod comprises at its ends clevis joints for taking up the forces, preferably adapted to transmit the forces along the axis of the connecting rod and to avoid generating forces that could bring about a deflection of the connecting rod, a deflection to which it is not adapted.
FIG. 1 represents a conventional connecting rod.
The connecting rod 1 comprises a central portion 10, the body of the connecting rod, substantially cylindrical, of the section that is more often than not circular, and of axis 14.
At each of its ends, the connecting rod comprises a zone 11a, 11b in which the external section of the connecting rod decreases progressively from the central portion 10 toward the end of the connecting rod. These end zones 11a, 11b are then designated as swagings.
At the end of each swaging, fixing elements 12a, 12b, for example clevis joints fitted with ball joints, are added and joined to the connecting rod for the connecting rod to be able to be assembled with a structure in whose strength it is required to participate.
The connecting rod is designed according to the forces to be transmitted and the central and generally hollow portion, as illustrated in the partial cutaway portion of the connecting rod in FIG. 1, and in the section of said figure, in order to distribute the resistant material of the connecting rod at the periphery of the body of the latter so that the connecting rod has a good resistance to buckling phenomena when the connecting rod is subjected to compression forces, without compromising the weight of the connecting rod.
The wall 13 of the central portion of the connecting rod 1 is produced, for at least aeronautical applications, more often than not using a light alloy based on aluminum or composite comprising long fibers, for example glass, Kevlar® or carbon fibers, held in a matrix of hardened organic resin, for example an epoxy resin.
These days, composites are being increasingly used because of a certain number of advantages:                favorable weight balance for a given resistance;        possibility of favoring the orientation of the fibers to take account of the preferred direction of the forces to be transmitted;        better fatigue behavior than metal.        
Various methods are known for producing such connecting rods in composite, either by methods using fibers pre-impregnated with resin before they are shaped, or by methods using fibers that are dry and then impregnated after they have been shaped during a resin transfer step following a method known as RTM.
In order to produce the composite part of the connecting rod, at least the body and the swagings, and possibly the end clevis joints, it is necessary to have a core or mandrel that is rigid enough to deposit and maintain the fibers, pre-impregnated or not, with the desired form until the resin of the composite has hardened, generally by polymerization during a hardening thermal cure.
When the resin has hardened, that is to say when the composite has reached a sufficient rigidity with respect to the desired resistance, it is generally preferred to remove the core which is of no further use and that can represent a not-inconsiderable weight or present physico-chemical characteristics, for example the release of fumes in case of fire, that are incompatible with the desired use of the connecting rod.
Because of its form, it is generally not easy to extract the core from the connecting rod.
The core must therefore be designed to allow it to be extracted, either by deformation or by destruction.
The cores that can be extracted by deformation are generally produced using elastomer, in one or more solid or hollow portions. Such cores have the advantage of being able to be used several times but their use remains complex for parts that have to be produced in relatively large quantities and they require particular precautions to obtain the form and the dimensions of the connecting rod because of their natural lack of rigidity.
The cores that are destroyed in order to be extracted are, for example, cores that can be chemically dissolved or melted by raising the material used to produce said core above a melting point. Such cores can be produced with well-controlled dimensions but they are often heavy and fragile to use and, by design, they can be used only once to produce a connecting rod.
The French patent published under the number 2705610, also corresponding to the U.S. Pat. No. 5,428,896, describes the production of a connecting rod using a single-usage core around which is produced a connecting rod of composite by filament winding of a pre-impregnated fiber.
After the composite has hardened, the core is dissolved in a solvent.
Such a method of producing a connecting rod, in addition to the problems intrinsic to the production, the handling and the elimination of the core, stems from the fact that the fibers must be deposited with certain angles relative to the axis of the connecting rod during the winding operation and that, because of these angles to be observed, it is not possible to orient fibers in the most theoretically optimal manner, that is to say parallel to the axis of the connecting rod.